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Mask Attack

Description

Try all combinations from a given keyspace just like in Brute-Force attack, but more specific.

Advantage over Brute-Force

The reason for doing this and not to stick to the traditional Brute-Force is that we want to reduce the password candidate keyspace to a more efficient one.

Here is a single example. We want to crack the password: Julia1984

In traditional Brute-Force attack we require a charset that contains all upper-case letters, all lower-case letters and all digits (aka “mixalpha-numeric”). The Password length is 9, so we have to iterate through 62^9 (13.537.086.546.263.552) combinations. Lets say we crack with a rate of 100M/s, this requires more than 4 years to complete.

In Mask attack we know about humans and how they design passwords. The above password matches a simple but common pattern. A name and year appended to it. We can also configure the attack to try the upper-case letters only on the first position. It is very uncommon to see an upper-case letter only in the second or the third position. To make it short, with Mask attack we can reduce the keyspace to 52*26*26*26*26*10*10*10*10 (237.627.520.000) combinations. With the same cracking rate of 100M/s, this requires just 40 minutes to complete.

Disadvantage compared to Brute-Force

There is none. One can argue that the above example is very specific but this does not matter. Even in mask attack we can configure our mask to use exactly the same keyspace as the Brute-Force attack does. The thing is just that this cannot work vice versa.

Masks

For each position of the generated password candidates we need to configure a placeholder. If a password we want to crack has the length 8, our mask must consist of 8 placeholders.

  • A mask is a simple string that configures the keyspace of the password candidate engine using placeholders.
  • A placeholder can be either a custom charset variable, a built-in charset variable or a static letter.
  • A variable is indicated by the ? letter followed by one of the built-in charset (l, u, d, s, a) or one of the custom charset variable names (1, 2, 3, 4).
  • A static letter is not indicated by a letter. An exception is if we want the static letter ? itself, which must be written as ??.

Output

Optimized due its partially reverse algorithms, password candidates are generated in the following order:

aaaaaaaa
aaaabaaa
aaaacaaa
.
.
.
aaaaxzzz
aaaayzzz
aaaazzzz
baaaaaaa
baaabaaa
baaacaaa
.
.
.
baaaxzzz
baaayzzz
baaazzzz
.
.
.
zzzzzzzz

NOTE: This shows that the first four letters are increased first and most often. The exact number however can vary, especially in a smaller keyspace, but it is fixed until a keyspace has been scanned completly.

NOTE: If you use oclHashcat you can press “s” while cracking to see the progress. You also see a number of '*' chars in the “Plain.Text” section. The number of '*' chars tell us how many chars it actually uses in the current attack. Range can go from one to four.

Built-in charsets

  • ?l = abcdefghijklmnopqrstuvwxyz
  • ?u = ABCDEFGHIJKLMNOPQRSTUVWXYZ
  • ?d = 0123456789
  • ?s = !”#$%&'()*+,-./:;⇔?@[\]^_`{|}~
  • ?a = ?l?u?d?s

Custom charsets

All hashcat derivates have four commandline-parameters to configure four custom charsets.

--custom-charset1=CS
--custom-charset2=CS
--custom-charset3=CS
--custom-charset4=CS

These commandline-parameters have four analogue shortcuts called -1, -2, -3 and -4. You can specify the chars directly on the command line or use a so-called hashcat charset file (plain text file with .hcchr extension which contains the chars/digits to be used on the 1st line of the file). See examples below:

Examples

The following commands all define the same custom charset that consists of the chars “abcdefghijklmnopqrstuvwxyz0123456789” (aka “lalpha-numeric”):

-1 abcdefghijklmnopqrstuvwxyz0123456789
-1 abcdefghijklmnopqrstuvwxyz?d
-1 ?l0123456789
-1 ?l?d
-1 loweralpha_numeric.hcchr # file that contains all digits + chars (abcdefghijklmnopqrstuvwxyz0123456789)

The following command defines a charset that consists of the chars “0123456789abcdef”:

-1 ?dabcdef

The following command defines a full 7-bit ascii charset (aka “mixalpha-numeric-all-space”):

-1 ?l?d?s?u

The following command sets the first custom charset (-1) to russian language specific chars:

-1 charsets/special/Russian/ru_ISO-8859-5-special.hcchr

Example

The following commands creates the following password candidates:

command: -a 3 ?l?l?l?l?l?l?l?l
keyspace: aaaaaaaa - zzzzzzzz
command: -a 3 -1 ?l?d ?1?1?1?1?1
keyspace: aaaaa - 99999
command: -a 3 password?d
keyspace: password0 - password9
command: -a 3 -1 ?l?u ?1?l?l?l?l?l19?d?d
keyspace: aaaaaa1900 - Zzzzzz1999
command: -a 3 -1 ?dabcdef -2 ?l?u ?1?1?2?2?2?2?2
keyspace: 00aaaaa - ffZZZZZ
command: -a 3 -1 efghijklmnop ?1?1?1
keyspace: eee - ppp

Password length increment

A Mask attack is always specific to a password length. For example, if we use the mask ”?l?l?l?l?l?l?l?l” we can only crack a password of the length 8. But if the password we try to crack has the length 7 we will not find it. Thats why we have to repeat the attack several times, each time with one placeholder added to the mask. This is transparently automated by using the ”--increment” flag.

?l
?l?l
?l?l?l
?l?l?l?l
?l?l?l?l?l
?l?l?l?l?l?l
?l?l?l?l?l?l?l
?l?l?l?l?l?l?l?l

Hashcat charset files

Hashcat charsets files (file extension: .hcchr) are a convenient way to reuse charsets, define custom charsets and use the language-specific charsets shipped by hashcat.

These files can be used together with the –custom-charsetN= (or -1, -2, -3 and -4) parameter. Instead of providing all the charset directly on command line, the support for .hcchr files allows one to specify the path to the file:

-1 charsets/standard/German/de_cp1252.hcchr

It is important that .hcchr files are created with language specific file encodings (e.g. cp1252, ISO-8859-15 etc). For examples of content and encoding of .hcchr files, see the examples shipped with hashcat (e.g. [HASHCATROOT]/charsets/standard/Italian/).

Hint: use iconv and similar tools to convert the files to a language specific file encoding (if for instance created as UTF-8 file).

Hashcat mask files

Hashcat mask files (file extension: .hcmask) are files which contain custom charsets (optional) and masks (e.g. ?1?1?1?1?d?d) line-by-line. The advantage of using .hcmask files, which are plain text files, is that those files allow the hashcat user to have a set of predefined and well-working masks stored within a file (or several e.g. password policy specific files) where the lines contained in the hcmask file could for instance be sorted by increasing runtime and/or likelihood of matches*.

The general format of 1 single line in the .hcmask file is as follows:

[?1],[?2],[?3],[?4],[mask]

where the placeholders are as follows:

  • [?1] the 1st custom charset (–custom-charset1 or -1) will be set to this value, optional
  • [?2] the 2nd custom charset (–custom-charset2 or -2) will be set to this value, optional
  • [?3] the 3rd custom charset (–custom-charset3 or -3) will be set to this value, optional
  • [?4] the 4th custom charset (–custom-charset4 or -4) will be set to this value, optional
  • [mask] the mask which should (but does not need) to use the custom-charset defined by [?1], [?2], [?3] or [?4] and can use any additional predefined charset (?d, ?l, ?u, ?s, ?a) and can contain fixed chars too (example value: pass?1?d?d?2?l?l)

* see the PACK program and some example hcmask files shipped by hashcat (in the masks/ folder).

Notes: .hcmask files can be used together with -i (increment) parameter for brute-force mode. if [mask] contains ?1,?2,?3 or ?4 references without those being set via [?1], [?2], [?3], [?4], there might be unexpected outcomes. Therefore, this should be avoided if possible. If for instance [?2] was not set because not needed, the comma that would be normally following [?2] should also be omitted. See examples below:

Example

The following example.hcmask file contains some valid example lines which show how to use this feature:

?d?l,test?1?1?1
abcdef,0123,ABC,789,?3?3?3?1?1?1?1?2?2?4?4?4?4
company?d?d?d?d?d
?l?l?l?l?d?d?d?d?d?d

Charsets in hex

This can be done by some of the hashcat tools using the ”--hex-charset” flag.

Supported by

This attack is currently supported by:

Note: mask files are currently only supported by oclHashcat